Metal-semiconductor barrier devices such Schottky diode devices are widely used. For example, Schottky diodes are often integrated into digital logic circuits as fast switches. Also, discrete Schottky diodes are used as power rectifiers because, among other things, they sustain high currents at lower voltage drops compared to diffused pn-junction diodes. Additionally, Schottky diodes are used as variable capacitors that can be operated efficiently, for example, at microwave frequencies.
Schottky diodes are formed by placing a Schottky metal in direct contact with a semiconductor surface. Typical Schottky metals include chromium, platinum, and aluminum. One problem with Schottky diodes is that they exhibit, in general, higher leakage currents and lower breakdown voltages than theoretically predicted. This results in part from the presence of sharp contact edges, which results in severe electric field crowding when the device is under reverse bias conditions.
Manufacturers typically use a doped pn junction guard ring that substantially overlaps or covers the contact edges to lessen the electric field crowding effect. The guard ring is normally formed by the vertical diffusion of dopant of an opposite conductivity type to that of the semiconductor substrate, and the Schottky contact is then formed to contact those portions of the guard that are the most heavily doped. This approach is effective in reducing leakage current and increasing breakdown voltage. However, this approach results in a very high minority carrier injection during turn-on conditions because the guard ring is a parallel connected pn junction diode. This minority carrier injection significantly slows switching speeds and, in some integrated circuit applications, results in latch-up problems.
Accordingly, a need exists for a Schottky diode structure and method of manufacture that improves reverse breakdown voltage performance, overcomes the minority injection problem outlined above, is simple to integrate into existing integrated circuit process flows, and is cost effective.
For ease of understanding, elements in the drawing figures are not necessarily drawn to scale, and like element numbers are used where appropriate throughout the various figures to denote the same or similar elements. For clarity in the drawings, doped regions of device structures or regions may be illustrated as having generally straight line edges and precise angular corners. However, those skilled in the art understand that due to the diffusion and activation of dopants, the edges of doped regions generally are not straight lines and the corners are not precise angles, and typically are rounded. Although certain conductivity types (e.g., p-type and n-type) are disclosed below, it is understood that the present invention includes and is relevant to those devices where the conductivity types are reversed from those that are specifically described herein.